PT-IGBT is not designed to block high reverse voltage due to the n+ buffer layer. In addition to the previous planar gate IGBT structures, the trench-gate IGBT [30] was proposed and developed by some manufacturers, Figure 2.2c. The advantage of this IGBT type com-pared to the PT structure is the reduced saturation voltage, while keeping switching losses low. Advancements of all these IGBT types are under ongoing research.
TABLE 2.1:Comparison of PT and NPT IGBT
Characteristic PT NPT
Relation with Temperature NTC for low current,
PTC for current close to nominal mainly PTC
Conduction losses higher for NPT at current close
to nominal due to PTC
Short-circuit capability higher for NPT
Parallelization recommended only for high currents Possible
Blocking voltage capability low high
In this application, IGBT of Soft-Punch-Through [31] technology is used that belongs to the PT family, has a structure with thinner buffer layer than a PT device and exhibits lower conduction and switching-off power losses than a NPT device [32]. For IGBTs [27], the dependence of vce on temperature resembles the pin-diode′s that is, already, presented in the forward characteristic part of Figure 2.1b.
vce = 2kT
q ln( JFWd
2qDaniF (WLd
a)) + pLCHJCH
µEF FCox(Vgs− Vth)(2.3)
Similar to the diode case, focusing on the parameters that depend on temperature,ni has positive temperature coefficient and Da has negative temperature coefficient, whereas, from the second term of the sum, the mobility µEF F and the threshold voltageVth have positive temperature coefficients. Again, the behavior of the device with temperature change is a result of the balance between these temperature dependent parameters. For low cur-rent values, commonly up to the level of 1/4 of the nominal rating the device exhibits NTC behavior, whereas for higher current values it exhibits PTC behavior. Similar to the diode, the inflection point exists for the IGBT as well.
2.4 IGBT module structure
The module as a means of packaging for the IGBTs is used for applications at the medium-to-high power range and, usually, at a voltage level up to 6.5kV . For higher voltage
lev-els and high-power applications the series connection of press-pack devices is a well-established solution [2] . The power module provides protection of the semiconductor devices and facilitates the mounting on the cooling plate, as well as the interconnection of the power electronic converter parts.
A typical structure of a silicon (Si) IGBT module with integrated anti-parallel diodes is illustrated in Figure 2.4.
Figure 2.4: Power module structure.
A 3300V and 1200A open power module is illustrated in Figure 2.5. It consists of 6 sub-strates similar to the one in Figure 2.6. Each substrate consists of 4 IGBT chips and 2 diode chips. In Figure 2.6 the two springs at one side of the substrate correspond to the gate and emitter leads for the gate driver and the single spring at the left side is for the collector connection with the driver.
2.4. IGBT module structure
Figure 2.5: HiPak 3300 V/1200 A module (ABB property).
Figure 2.6: Substrate of HiPak 3300 V/1200 A module (ABB property).
The IGBT module is formed by the following layers that are illustrated in Figure 2.4 from top to bottom [33]:
• A thin aluminum metallization layer on top of the silicon chip assisting the chips
interconnection and the connection with the gate with the help of bond-wires, see, also, Figure 2.6
• Silicon chips as semiconductors
• Solder, usually an alloy of SnAg or Pb-Sn, for the connection of two consecutive layers. A replacement for the solder layers in some new power modules is the Ag sintered layer that exhibits a higher mechanical resistance and, also, ability to with-stand higher temperatures than the conventional solder layer.
The next three layers compose the Direct-Bond Copper (DBC) part of the module:
• Copper (Cu) for the heat conduction produced at the silicon chip layer
• Insulation layer, for the IGBT module of this work it is an Aluminum Nitride (AlN) ceramic that withstands a high breakdown voltage to electrically isolate the different substrates and the cooling plate
• Copper for the heat propagation facilitation after the insulation layer
• Solder
• Baseplate, although it does not exist in all modules, usually AlSiC or Cu depending on the application. Normally, it is the thickest layer with the greatest heat storage capability among the layers. At the baseplate layer, the material selection depends on the load profile. The AlSiC baseplate exhibits the best heat storage capability and it is designated for current profiles with long duration, commonly traction applica-tions.
• Thermal Interface Material (TIM) or thermal paste, for instance Si for the uniform contact between the baseplate and the heat sink or cooling plate surface to avoid air trapped in.
The three most important parameters for the material selection are the thermal conductiv-ity, the heat storage capability and the thermal expansion coefficient. Table 2.2 lists these three thermal characteristics for the IGBT module layers. The materials with the worst thermal conductivity are those of the TIM with a thermal conductivity of 1W/(mK), the DBC insulating and the solder layer. The interconnections inside the module among the silicon chips and the gate and emitter pads for the control of the device are, traditionally, implemented with Al bond wires.
2.4. IGBT module structure
TABLE 2.2:Thermal characteristics for each of the power module layers at 25◦C [33]
Material
AlSiC (75% SiC) 180 2223 7
Among the materials in the structure of the IGBT module, the three dominant conductive materials in terms of thickness are Si, Cu and Al. The thermal conductivity of Si is sig-nificantly dependent on temperature, whereas for the other two the variation of thermal conductivity with temperature is very small. Table 2.3 shows the change of thermal con-ductivity for the usual operation temperature range between 25C◦ to 125C◦ for the three materials [34].
TABLE 2.3: Variation of thermal conductivity for the three dominant conductive materials in the IGBT module
25C◦ 125C◦ Silicon 148 98.9
Copper 401 396
Aluminum 237 240
Focusing on Si, this relation is described as [26]
λ = 24 + 1.87 ∗ 106T−1.69 W
mK (2.4)
where the temperatureT is illustrated in Figure 2.7
Figure 2.7: Relation of thermal conductivity with temperature for Si.
The thermal resistance and thermal capacitance defining the thermal conductivity and the heat storage characteristic of the material, respectively, are expressed
Rth = d
λA (2.5)
Cth= cρdA (2.6)
whered is the thickness of the layer, ρ the Si density, λ the thermal conductivity of the material,A the chip surface area and c the specific heat capacity of the material.